Datasheet LTC7138 (Analog Devices) - 9

LTC7138
operaTion (Refer to Block Diagram) Start-Up and Shutdown
By connecting the FBO pin of a master LTC7138 to the VFB If the voltage on the RUN pin is less than 0.7V, the LTC7138 pin of one or more slave LTC7138s, the output currents enters a shutdown mode in which all internal circuitry is can be combined to source 400mA times the number of disabled, reducing the DC supply current to 1.4µA. When the LTC7138s. voltage on the RUN pin exceeds 1.21V, normal operation of
Dropout Operation
the main control loop is enabled. The RUN pin comparator has 110mV of internal hysteresis, and therefore must fall When the input supply decreases toward the output sup- below 1.1V to disable the main control loop. ply, the duty cycle increases to maintain regulation. The P-channel MOSFET switch in the LTC7138 allows the duty An internal 1ms soft-start function limits the ramp rate of cycle to increase all the way to 100%. At 100% duty cycle, the output voltage on start-up to prevent excessive input the P-channel MOSFET stays on continuously, providing supply droop. If a longer ramp time and consequently less output current equal to the peak current, which is greater supply droop is desired, a capacitor can be placed from the than the maximum load current when not in dropout. SS pin to ground. The 5µA current that is sourced out of this pin will create a smooth voltage ramp on the capacitor.
Input Voltage and Overtemperature Protection
If this ramp rate is slower than the internal 1ms soft-start, then the output voltage will be limited by the ramp rate on When using the LTC7138, care must be taken not to exceed the SS pin. The internal and external soft-start functions any of the ratings specified in the Absolute Maximum Rat- are reset on start-up, after an undervoltage or overvoltage ings section. As an added safeguard, however, the LTC7138 event on the input supply, and after an overtemperature incorporates an overtemperature shutdown feature. If the shutdown. junction temperature reaches approximately 180°C, the LTC7138 will enter thermal shutdown mode. The power
Peak Inductor Current Programming
switch will be turned off and the SW node will become high impedance. After the part has cooled below 160°C, it will The peak current comparator nominally limits the peak restart. The overtemperature level is not production tested. inductor current to 610mA. This peak inductor current can be adjusted by placing a resistor from the ISET pin to The LTC7138 additionally implements protection features ground. The 5µA current sourced out of this pin through which inhibit switching when the input voltage is not within the resistor generates a voltage that adjusts the peak cur- a programmable operating range. By use of a resistive rent comparator threshold. The valley current threshold divider from the input supply to ground, the RUN and tracks the peak current threshold setting, and is typically OVLO pins serve as a precise input supply voltage moni- 60% of the peak current. tor. Switching is disabled when either the RUN pin falls below 1.1V or the OVLO pin rises above 1.21V, which can During sleep mode, the current sourced out of the ISET pin be configured to limit switching to a specific range of input is reduced to 1µA. The ISET current is increased back to 5µA supply voltage. Furthermore, if the input voltage falls below on the first switching cycle after exiting sleep mode. The 3.5V typical (3.8V maximum), an internal undervoltage ISET current reduction in sleep mode, along with adding detector disables switching. a filtering network, RISET and CISET, from the ISET pin to ground, provides a method of reducing light load output When switching is disabled, the LTC7138 can safely sustain voltage ripple at the expense of lower efficiency and slightly input voltages up to the absolute maximum rating of 140V. degraded load step transient response. Input supply undervoltage or overvoltage events trigger a soft-start reset, which results in a graceful recovery from For applications requiring higher output current, the an input supply transient. LTC7138 provides a feedback comparator output pin (FBO) for combining the output current of multiple LTC7138s. 7138f For more information www.linear.com/LTC7138 9 Document Outline Features Applications Description Typical Application Absolute Maximum Ratings Pin Configuration Order Information Electrical Characteristics Typical Performance Characteristics Pin Functions Block Diagram Operation Applications Information Typical Applications Package Description Typical Application Related Parts